This is a common upgrade issue with the built in web server.
This only applies if you are using the built in web server, not IIS.
In the elevated command prompt, run:
netsh http add urlacl url=http://*:<<port>>/ user=<<user>>
where <<port>> is the port you have set the SQL Monitor web service to use, and <<user>> is the user the web service is running as.
You will then need to restart the SQL Monitor BaseMonitor Service, and the SQL Monitor Web Service.